In recent years, in view of energy saving and miniaturization, light emitting devices (LED flashes) using a light-emitting diode (hereinafter also referred to as “LED”) as a light source have been increasingly used for a light emitting device in an imaging camera. For example, light emitting devices using a combination of an LED and a Fresnel lens are known.
In general, the shape of the imaging region of an imaging camera is quadrangle. Accordingly, in order to obtain a clear image, it is preferable that a light-emitting device illuminate an irradiation region in a quadrangular shape. Therefore, a Fresnel lens used in a light-emitting device of an imaging camera is required to uniformly and efficiently irradiate the quadrangular irradiation region with light emitted from a light emitting element. Conventionally, various Fresnel lenses for illuminating a quadrangular irradiation region have been proposed (see, for example, PTLS 1 and 2).
FIG. 1A is a perspective view of Fresnel lens 10 disclosed in PTL 1. Fresnel lens 10 illustrated in FIG. 1A has the same function as cylindrical lens 20 illustrated in FIG. 1B. As illustrated in FIG. 1A, in Fresnel lens 10, a plurality of grooves 12 each having a rectangular shape in plan view are concentrically formed in a spaced relationship.
FIG. 1C is a plan view of Fresnel lens 30 disclosed in PTL 2. As illustrated in FIG. 1C, Fresnel lens 30 includes first condenser lens 40 disposed at a center portion, and second condenser lens 50 disposed on the outer side of first condenser lens 40. First condenser lens 40 is divided into three regions. In both end regions, a plurality of grooves 42 each having a semicircular shape in plan view are concentrically formed in a spaced relationship. In a center region, a plurality of cylindrical lenses 44 are formed. In second condenser lens 50, a plurality of grooves 52 each having a circular shape in plan view are concentrically formed in a spaced relationship.